1. Field of the Invention
The present invention relates to an apparatus and method for receiving transmitted digital video signals and operation of digital devices/terminals.
2. Description of Related Art
Use of digital video signals such as digital television (DTV) services via terrestrial broadcasting has gained momentum worldwide recently. One of the attractive features of DTV is its capability to deliver content to mobile terminals or handheld devices. For a mobile DTV device, especially a handheld one, however, low power consumption is desirable for obtaining reasonable usage and standby cycles. Mobility is another requirement such that access to services is possible not only at indoor and outdoor locations but also when the user is on the move, for example, when in a vehicle. To some extent, these two requirements are mutually exclusive. In order to provide high quality services in a highly mobile environment, the devices are implemented with sophisticated signal processing algorithms for mitigating adverse transmission channel effects, which, of course, result in considerably increased power consumption. Therefore, the application of effective power consumption reduction schemes in the implementation of a mobile and/or handheld digital television terminal/device is highly desirable.
Various schemes for power consumption reduction have been proposed in the area of digital terrestrial broadcasting. A particularly well-known scheme is the so-called time-slicing technique adopted in the European Digital Video Broadcasting-Handheld (DVB-H) specification, described in more detail in “Digital video broadcasting (DVB); transmission system for handheld terminals (DVB-H)”, ETSI EN 302 304 V1.1.1 (2004-11), “Digital video broadcasting (DVB); DVB specification for data broadcasting”, ETSI EN 301 192 V1.4.1 (2004 November), “Digital video broadcasting (DVB); DVB-H implementation guidelines”, ETSI TR 102 377 V1.1.1 (2005 February), European Telecommunications Standards Institute, and also in G. Faria, J. A. Henriksson, E. Stare, and P. Talmola, “DVB-H: Digital Broadcast Services to Handheld Devices,” Proc. IEEE, Vol. 94, January 2006, pp. 194-209. The DVB-H system is defined based on its parent Digital Video Broadcast-Terrestrial (DVB-T) standard for fixed and mobile/handheld reception of digital TV signals. The use of time-slicing is mandatory in DVB-H and it can reduce the average power in the receiver front-end significantly—up to 90% to 95% in comparison with its DVB-T counterpart.
The power saving made possible by the time-slicing technique in DVB-H comes from the fact that essentially only those parts of the moving picture experts group (MPEG) transport stream (TS) which carry the currently selected data of the service have to be processed. Thus, service multiplexing can be performed solely in a time-division multiplex (TDM). The data of one particular service are therefore not transmitted continuously—as shown in FIG. 1a—but in compact periodical bursts with interruptions in between—as shown in FIG. 1b. This type of signal can be received time-selectively; the terminal/device synchronises to the bursts of the selected service but switches to a power-save mode during an intermediate time period when other services are being transmitted.
To perform the time-slicing in a DVB-H system properly, bursts entering the receiver have to be buffered and read out of the buffer at the service data-rate. The amount of data contained in one burst needs to be sufficient for bridging the power-save period of the front-end. The position of the bursts is signaled in terms of the relative time difference between two consecutive bursts of the same service. Practically, the duration of one burst (on-time 2 in FIG. 1b) is in the range of several hundred milliseconds whereas the power-save time (off-time 4 of FIG. 1b) may amount to several seconds. A lead time for powering up the front end, for resynchronisation and so on has to be taken into account; this time period is assumed to be less than 250 ms in DVB-H case.
In general, and referring again to FIG. 1, the TDM based power saving can be measured as the ratio of the power-save time between bursts, relative to the on-time 2 required for the reception of an individual service, i.e.,
                    η        ≈                              [                          1              -                                                                                          S                      b                                        /                                          C                      b                                                        +                                      t                    s                                                                                        S                    b                                    /                                      C                    l                                                                        ]                    ×          100          ⁢          %                                    (        1        )            Where Sb is the burst size in bits, Cb is the burst data-rate in bit-per-second (bps), C1 is the expected service data-rate (continuously transmitted with lower rate) in bps of a handheld device, while ts is the lead time in seconds.
In a DVB-H system, the burst size Sb=2 Mbits, the maximum burst transmission rate is around Cb=10 Mbps, and the required lead time is about ts=250 ms. In this case, the off-time 4 is around 4 s. Thus, for a typical service data-rate of C1=384 kbps, about η=91% power saving can be achieved. This makes it feasible for a handheld device to provide a DTV service.
A similar power saving scheme has also been proposed for use in the Digital Multimedia Broadcasting-Terrestial (DMB-T) system, which is a candidate for becoming or partially becoming the digital terrestrial television (DTT) broadcast standard in some countries: e.g. China, see China Patent No. 00123597.4, publication date: Mar. 21, 2001, and also Z-X. Yang, M. Han, C-Y. Pan, J. Wang, L. Yang, and A-D Men “A Coding and Modulation Scheme for HDTV Services in DMB-T,” IEEE Trans. Broadcasting, Vol. 50, March 2004, pp. 26-31. The technique tailored for power saving in DMB-T is called frame-slicing, which is disclosed in China Patent Application No. 200410009721.5, publication date: Oct. 29, 2004. A significant difference between time-slicing and frame-slicing is that the former is realised in the link layer (i.e., the layer above the physical layer) whereas the latter is realised purely in the physical layer.
As shown in FIG. 2, DMB-T adopts a hierarchical frame structure 6. A basic frame element is called a Signal Frame 8. The Frame Group 10 is defined as a group of signal frames 8 with the first frame specially defined as Frame Group Header 12. The Super Frame 14 is defined as a group of Frame Groups 10. The top of the frame structure is called a Calendar Day Frame 16. The physical channel is periodical and synchronised with the absolute time as depicted by time markers 18a, 18b. 
One of the features which differentiate DMB-T from other DTT devices is its adoption of the time-domain synchronous multi-carrier transmission technique referred to as TDS-OFDM. As depicted in FIG. 2, a signal frame 8 consists of two parts: Frame Sync 20 and Frame Body 22. The TDS-OFDM inserts pseudo-random number (PN) sequences 24 and their cyclical extensions as the guard intervals, which also serve for synchronisation and channel estimation. This time-domain synchronous technique can achieve fast frame and symbol timing acquisition with the theoretical lead time, ts, of only about 2 ms, which is desirable for TDM-based power saving schemes, as can be seen from equation (1). The signal frame 8 also comprises an IDFT Block 26.
As shown in FIG. 2, the frame-slicing power saving scheme for DMB-T is to form a number of frame slices 28, each with a certain number of successive signal frames 8 which belong to the same frame group 10. Typically, a frame slice 28 consists of four signal frames 8. The frame-slicing scheme is different from the time-slicing scheme, which is purely dependent on the arrangement for on-off transmission in the link layer, whereas the frame-slicing scheme is physical layer based. This gives some flexibility in controlling the burst period and the power-saving period. Obviously, the burst size can be chosen to be the size of a frame slice 28. When a signal frame 8 is of 625 μs long, the duration of a frame slice 28 is 2.5 ms. In this case, the burst data-rate of Cb=24 Mbps, the burst size is found to be Sb=60 Kbits. Taking into consideration a lead time of ts=2 ms and following equation (1), one may find that, in this case, for a service data-rate of C1=384 kbps, approximately η=97% power saving can be achieved.
From the above discussion, it is apparent that both time-slicing and frame-slicing are passive schemes which gain power savings at the price of decreased service data rates.